A plate at a distance of 0.03 mm from a fixed plate moves at 0.8 m/s and requires a force of 1.50 N/m2 area of plate. Determine the dynamic viscosity of liquid between the plates.

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SSC JE Civil 15 Nov 2022 Shift 1 Official Paper
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  1. 50.25 × 10−6 N-S/m2
  2. 56.25 × 10−6 N-S/m2
  3. 6.25 × 10−6 N-S/m2
  4. 66.25 × 10−6 N-S/m2

Answer (Detailed Solution Below)

Option 2 : 56.25 × 10−6 N-S/m2
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Detailed Solution

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Concept:

According to newton's law of viscosity-

This law states that the shear stress (\(\tau\)) on a fluid element layer is directly proportional to the rate of shear strain. The constant of proportionality is called the coefficient of viscosity.

\(\tau=\mu\, .({du\over dy})\)

Where \(\tau=\) shear stress \(={Force\over Area}\)

\(\mu=\) dynamic viscosity

\({du\over dy}=\) velocity gradient

Calculation:

 F5 Vinanti Engineering 21.12.22 D5

Given data:

Distance between plates (y) = 0.03 mm or 3 × 10-5 m

Velocity of moving plate (\(v\)) = 0.8 m/s

Force per unit area on plate (F/A) = 1.5 N/m2

Dynamic viscosity (μ) =?

\(Shear\, stress(\tau)=\mu× ({v\over y})\)

\(1.5=\mu× ({0.8\over 3\times 10^{-5}})\)

\(\mu={1.5\times 3\times 10^{-5}\over 0.8}\)

\(\mu=5.625\times 10^{-5}\, N-s/m^2\)

\(\mu=56.25\times 10^{-6}\, N.s/m^2\)

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